This invention relates to a multipolar dc motor having an improved pigtail arrangement, and more specifically, to brush pigtails specially arranged in a dc motor so as to improve durability against vibration, moisture, and the like.
Japanese Utility Model Application preliminary publication No. sho 58-150355 discloses a conventional multipolar dc motor which includes a pair of positive brushes and a pair of negative brushes. These four brushes are circumferentially disposed around a commutator at various angular intervals, and are in sliding electrical contact with the commutator. All the brushes are movably received in their respective brush holders which are mounted on an annular brush stay. The positive brushes are approximately 120 degrees apart, and then the negative brushes are about 120 degrees apart. Each of the positive brushes is diametrically opposed to a negative brush. A positive brush and the adjacent negative brush are approximately 60 degrees apart, and the other positive brush and the adjacent negative brush are about 60 degrees apart. Springs which urge the positive brush against the commutator are disposed respectively in the smaller spaces between the positive and negative brushes, while springs which urge the negative brushes are disposed in the larger space between the negative brushes. Pigtails, which are lead wires for connecting the positive brushes to a power source, extend respectively from the positive brushes, being routed into the larger space between the positive brushes, and pass out of the motor case through an insulating member called "grommet" which engages with the motor case.
In the conventional dc motor mentioned above, however, the pigtails are not fixed to any part of the motor but are only loosely restrained by the grommet. Therefore, if the motor is installed in an automotive vehicle, for example, and thus is subjected to vibration, particular portions of the pigtails, such as those portions adjacent to the grommet, undergo repeated stresses. This often results in the disconnection of the pigtails in an unexpectantly short period.
Furthermore, although the aforementioned Japanese publication discloses a proper wiring arrangement, utilizing the larger space, for the pigtails of the positive brushes, it discloses no wiring arrangement for the pigtails connected to the negative brushes. Should these negative pigtails be extended into the smaller spaces, they can easily contact the springs for the positive brushes, causing a shortcircuit. On the other hand, if the negative pigtails pass out of the motor case through the larger space between the negative brushes, these pigtails must be lengthened and routed around the springs for the negative brushes to avoid interference with the springs. Such lengthened pigtails, however, tend not only to interfere with the movement of the negative brushes, but also tend to rub against the springs due to the vibration. These tendencies will decrease the reliability and durability of the motor.
Moreover, since each of the pigtails is formed of many filament wires twisted or braided together, a problem can arise due to capillary action, when a liquid such as rain water and oil is wicked into the motor case through the pigtails which pass out of the motor case therethrough. When the brushes are wet due to such rain water or oil, the abrasion of the brushes will be accelerated.
Lastly, in conventional motors, the positioning of the brush stay relative to the rear bracket of the motor case is not taken into account. The brush stay must be accurately positioned on the rear bracket to mount the stay on the bracket by means of machine screws. However, it is difficult in the conventional motors to align the axes of the threaded holes of the stay respectively with those of the through-holes of the bracket. That difficulty has lowered workability during the assembly of the motors.